1. Field of the Invention
The present invention relates to a point-to-multipoint optical communication system with a standby interface that can be used in case of failure of another interface.
2. Description of the Related Art
The rapid build-up of an installed base of asymmetric digital subscriber lines (ADSL) and other broadband subscriber communication lines has led service providers to begin providing services that require high-speed communication. To expand these services, a still faster and more stable broadband infrastructure will be necessary, which makes the extension of optical fiber to the home (FTTH) increasingly attractive. A known means of providing FTTH service at a low cost is the passive optical network (PON), in which a plurality of users share a single optical fiber. The term PON designates a network with a star topology that uses a passive optical coupler to form a point-to-multipoint communication system.
FIG. 1 shows the general scheme of a point-to-multipoint communication system using PON technology. An optical line terminal (OLT) at a telephone switching office is connected by optical fiber to a wide area network (WAN) 600. The OLT 100 has a PON interface (IF) card that uses a time-division technique to multiplex data received from the WAN 600 into downstream frames for transmission to a plurality of subscribers. The multiplexed data frames are transmitted on an optical fiber 200 to an optical coupler, which passively branches all the data frames onto a plurality of optical fiber subscriber lines 201, 202, 203 leading to optical network units (ONU) 401, 402, 403 installed on subscriber premises. Each ONU uses an identifier attached to the frames to select the frames addressed to it and converts these frames to electrical signals. The electrical signals are sent to subscriber terminal equipment 501, 502, 503. The identifiers are assigned by the OLT 100 in a registration process carried out when the ONUs are installed. The OLT manages the identifiers to make sure that no two ONUs connected to it have the same identifier.
In the upstream direction, an ONU converts an electrical data signal received from subscriber terminal equipment to upstream optical data frames that are transmitted through the coupler 300 to the OLT 100 at timings designated by the OLT 100. The OLT 100 controls the timings and the amount of data per frame so that frames from different ONUs do not collide in the optical coupler 300. The OLT 100 receives the upstream frames and sends them to the WAN 600.
As shown in FIG. 2, a plurality of PON interface cards may be installed in a single OLT 101. These cards (PON IF #1 to PON IF #M) are connected to respective 1:N optical couplers 300-1 to 300-M, each of which can branch an incoming signal onto N subscriber lines, enabling the OLT 101 to connect with up to M×N ONUs (ONU #1-1 to ONU #1-N, ONU #2-1 to ONU #2-N, . . . , ONU #M-1 to ONU #M-N), where M and N are integers greater than unity. By increasing the number of ONUs connectable to a single OLT, this scheme uses system resources more efficiently.
This type of point-to-multipoint system may have built-in redundancy, enabling it to continue communication even if a fault occurs on a data transmission path between the OLT in the telephone switching office and a subscriber's ONU. That is, the system may include active data transmission paths which are used normally, and one or more standby data transmission paths to which it can switch over when a fault occurs on one of the active paths.
Japanese Patent Application Publication No. 2005-328294 discloses a dual OLT (master device) with two independent PON interfaces connected by different optical fibers to the same coupler and thus to the same ONUs (slave devices). If the active interface fails, communication is immediately switched over to the standby interface, without changing the phase of the optical signals received at the ONUs. The faulty interface can then be replaced while the good interface continues to operate. This disclosure, however, does not contemplate the situation in FIG. 2, in which an OLT has multiple active PON interfaces.
If one of the PON interfaces in FIG. 2 fails, it would be desirable to have a standby interface to which the failed interface could be switched over without affecting the other active interfaces. A problem is that each PON interface operates according to its own control information, specifying such parameters as the round trip time between the OLT and each ONU, by which it controls transmissions so as to avoid collisions. Differences between the control information held by different PON interfaces would make it difficult for a single standby interface to be capable of immediately replacing any active interface that failed.
Providing a separate standby PON interface for each active PON interface would greatly limit the number of active interfaces that could be installed in a single OLT. In view of the expected high demand for FTTH service, there is a need for a more efficient form of redundancy.